Yet another "me too" I'm afraid. I have exactly the same symptoms on my new 2.66GHz 15' MacBook Pro with the stock 320GB HD. I was fine for the first two weeks, then the unexplained spinning beachball usually in Safari or Finder, but I have seen it in other programs. I thought it was because I had installed Greasekit on my system so I temporarily removed it, but the problem persisted. I went ahead and reinstalled it. Interestingly, I haven't experienced the problem in about a week, but my computer has been on a stand connected to my 30in Cinema Display that whole time. I don't hear the drive head parking while its on the desk, but when I use it on my lap I have noticed that my new MacBook Pro is much more sensitive to the sudden motion sensor than my previous early 2008 Penryn model. The slightest motion will cause the drive to click. It would be nice to get this fixed for good.
BTW: There's another thread where people are noticing that this started happening for them after they installed the new SATA II firmware upgrade. Now that I think about it, I may have started experiencing the lockup problems after I installed the upgrade on my MacBook Pro.